(i) Field of the Invention
This invention relates to a method and apparatus for separating liquid components from a liquid mixture.
(ii) Prior Art
Until relatively recently, the use of hydrocyclones, or cyclone separators, in separating technology was thought to be limited to separation of components from mixtures where there were substantial differences in specific gravity of components to be separated. In particular, hydrocyclones were regarded as being of only limited usefulness in separating liquid components one from another in a liquid mixture, in view of the relatively small differences in specific gravity existing as between many liquids. More recently, however, cyclone separators capable of separating liquid components one from the other within a liquid mixture have become more commonly used. In particular, U.S. Pat. No. 4,237,006 and U.S. Pat. No. 4,576,724 describe cyclone separators capable of effective separation of liquid components, and these separators have found commercial application in oil processing technology, particularly in the separation of residual oil from a mixture predominantly comprising water. Such a mixture is produced as a by-product of initial separation of oil from formation liquid initially recovered from an oil well. Similar technology has subsequently been applied, using cyclone separators, for separation of oil and water components from admixtures thereof where there are significant quantities of water, unlike the last described application, where, generally speaking, the inlet liquid has only a relatively small amount of oil present therein.
In the arrangements described in the aforementioned U.S. patent, the cyclones separators are of particular form. More particularly, the separating chamber is in each case in the form of an axially extending surface of revolution having three portions, the three portions having diameters, at their largest diameter ends, of d.sub.1, d.sub.2 and d.sub.3 respectively and lengths l.sub.1, l.sub.2 and l.sub.3 respectively. The three portions may be cylindrical in form in the sense that they have sides which are parallel to the axis of the separating chamber, or they may have a taper. It has been found particularly advantageous that the second portion should have such taper. Furthermore, a flow smoothing tapered portion may be interposed between the first and second portions. The separating chamber has one or more inlets at the first portion arranged for providing inflow of a mixture to be separated with a tangential flow component. There is an axially arranged underflow outlet at the end of the third portion remote from the first portion and an axially arranged overflow outlet at the end of the first portion remote from the third portion. More particularly, the separator of U.S. Pat. No. 4,237,006 is characterised by the following: ##EQU2## where A.sub.1 is the total inlet area of the or each inlet.
The arrangement described in U.S. Pat. No. 4,576,724 is generally similar to the arrangement in U.S. Pat. No. 4,237,006 save that the following relationship is in the latter said to apply. EQU d.sub.0 /d.sub.2 &lt;0.1
the second portion being tapered, such as with a half angle of taper in the range of 20' to 2.degree.
Generally speaking, cyclone separators, for effective separation of oil-water mixtures, should be of form similar to the above, but it has now been found that constructions of more general form may be suitable. In particular, it is not absolutely essential that the three separately defined portions as above described be provided. For example, arrangements which exhibit a continuous taper change possibly with portions of constant diameter interposed therewithin or at either or both ends may be utilised. In such cases, the parameter d.sub.2 above described is more generally defined as being the diameter at the axial position Z.sub.2 where the following condition is first met along the axis of the separator from an inlet plane (at which z=o) defined later: ##EQU3## for all z&gt;z.sub.2 where d is the cyclone diameter at z,
Furthermore, the area A.sub.i may be more generally defined as follows: ##EQU4## where A.sub.ix is the projection of the cross sectional area of the x.sup.th inlet measured at entry to the cyclone in the plane parallel to the cyclone axis which is normal to the plane, also parallel to the cyclone axis, which contains the tangential component of the inlet centre line, and the diameter d.sub.1 above referred to may be replaced by a parameter d.sub.i defined as follows; ##EQU5## where d.sub.ix is twice the radius at which flow enters the cyclone through the x.sup.th inlet, (i.e., twice the minimum distance of the tangential component of the inlet centre line from the axis).
The inlet plane is defined as the plane perpendicular to the axis of the cyclone at the mean axial position of the weighted areas of the inlets such that the injection of angular momentum into the hydrocyclone is equally distributed axially about it and is thus such that ##EQU6## wherein Z.sub.x is the axial position of the centre line of the x.sup.th inlet.
The diameter d.sub.3 may also be more generally defined as the diameter at z.sub.3 where d/d.sub.3 &gt;0.98 for all z&gt;z.sub.3. Then, further, .alpha. is defined as ##EQU7##
In any event, it should be noted that the parameter ##EQU8## abovementioned is similar to, but not identical with, a further parameter which has been determined as being relevant to the design of cyclone separators of the kind in question. That parameter is herein termed the swirl coefficient and is herein designated `S` and is defined, for the purposes of this specification, as follows: ##EQU9##
In particular constructions described in the above described U.S. patent specifications, the diameter d.sub.2 is one half the diameter d.sub.1. In such case, the swirl coefficient S, as above described has values in the range 5 to 12.5. While it has been found that values within this range are suitable for many applications, it has now been determined that, for certain specific circumstances, optimum operation or near optimum is not provided by use of a cyclone separator exactly as contemplated in the two U.S. patents. More particularly, certain mixtures have proven difficult to adequately separate by use of such separators. These include mixtures where there is a predominance of water, the water comprising a "continuous" phase, and the oil being present in the form of droplets therewithin so as to comprise a "disperse" phase, and also include such mixtures where the mean oil droplet size is relatively small.